Field of the Invention
The present invention is suitable for a projector that projects an image in a personal computer or the like onto a screen in an enlarged manner, and in particular relates to an optical system that can rotate an image projected on a screen and to an optical apparatus including such an optical system.
Description of the Related Art
To date, a projection optical system for rotating an image on a screen of a projector has been proposed. Japanese Patent Laid-Open No. 2000-78616 discloses a projection-type display apparatus. In the projection-type display apparatus, an image rotating prism constituted by a Dove prism for reflecting light once that is rotatable about an optical axis is disposed between a projection lens and a display element, and thus an image projected on a screen can be rotated.
According to Japanese Patent Laid-Open No. 2010-122485, a Pechan prism for reflecting light five times is disposed between a projection lens and a relay lens unit, and thus an image on a screen can be rotated. In addition, according to Japanese Patent Laid-Open No. 2005-24573, one of two reflection mirrors disposed outside a projector is rotated about an optical axis of a projection lens, and thus an image on a screen is rotated.
As in the existing techniques described above, projection optical systems for rotating an image on a screen have been proposed. However, when an image rotating prism for rotating a projected image about an optical axis is embedded inside a projection optical system, an optical path for disposing the image rotating prism becomes excessively long.
Specifically, according to Japanese Patent Laid-Open No. 2000-78616, the optical path length necessary for disposing the Dove prism is approximately 4.4 times the effective diameter of the one of an incident surface of the prism and an exit surface thereof that has a greater effective diameter. In addition, according to Japanese Patent Laid-Open No. 2010-122485, the optical path length necessary for the Pechan prism is 4.6 times the effective diameter of the one of an incident surface of the Pechan prism and an exit surface thereof that has a greater effective diameter.
When an optical path necessary for disposing an image rotating prism is long, the total length of the projection optical system increases. Furthermore, when an optical path within a prism is long, this leads to a problem in that the diameter of the front lens in the projection optical system needs to be increased. In addition, due to the design of a projection optical system, a space that is free of any lens needs to be secured within the optical system, and this may impose considerable constraint on the design, leading to a problem in that good optical performance cannot be achieved.
Furthermore, according to Japanese Patent Laid-Open No. 2005-24573, when the projection optical system has a wide angle, this leads to a problem in that the size of a reflection mirror is increased excessively. In a case of the front projection in which the projector and a viewer viewing an image projected on a screen are located on the same side relative to the screen, when the size of the reflection mirror is excessively large, this leads to a problem in that the reflection mirror may cover the screen and hinder the viewing.
As described above, when the optical systems that have been proposed to date are to be applied in specific cases, the optical systems require a long optical path and are thus unable to be put into practical use.